Make auth provider autologin modular and implement it for all OAuth2 adapters

Summary:
Ref T10785. Around the time we launched Phacility SAAS we implemented this weird autologin hack. It works fine, so clean it up, get rid of the instanceof stuff, and support it for any OAuth2 provider.

(We could conceivably support OAuth1 as well, but no one has expressed an interest in it and I don't think I have any OAuth1 providers configured correctly locally so it would take a little bit to set up and test.)

Test Plan:

• Configured OAuth2 adapters (Facebook) for auto-login.
• Saw no config option on other adapters (LDAP).
• Nuked all options but one, did autologin with Facebook and Phabricator.
• Logged out, got logout screen.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10785

Differential Revision: https://secure.phabricator.com/D16060